The catalysts based on vanadium, supported on a siliceous matrix, are commonly utilized for the oxidation of sulphur dioxide to sulphur trioxide; see for example British patent Nos. 808,630; 898,985; 972,402 and 1,520,336; U.S. Pat. Nos. 3,448,061 and 4,485,190 and European patent Nos. 47,540 and 151,823, the contents of which is an integral part of the present description. As a siliceous matrix, use was made so far of diatomaceous earth and of finely subdivided silica gel; but good results are obtainable also with alpha quartz (cristobalite), silicalites, vanadium silicalites or titanium silicalites. As to the meaning of these terms, reference is made to Italian patent publication 22,220 A/87 and to British patent No. 2,024,790. These catalysts can be approximately represented by the raw formula (I): EQU V.sub.x K.sub.y Na.sub.z O.sub.w S.sub.t (I)
where x, t, w, z and t are broadly varying indexes (depending on the operative conditions and on the ageing degree) and where z can also be zero. Also iron (Fe) and other elements (Cs, Al, Mn, Mg and so on) are often present. Still further elements (As, Cl, F, Se and so on), even if in a very low amount, surely act as poisons for this catalysis. The catalyst, utilizable in both fixed and fluidized beds, contains vanadyl-alkali metal sulphates and pyrosulphates, the concentration of which varies as a function of time. The yields obtainable with these catalysts are high, especially if use is made of a series of more catalyst layers (at least 3 or 4) of the axial or radial type. Generally the catalyst is prepared by impregnating a diatomaceous earth or a different siliceous carrier (average diameter=about 1 to 40 micrometers), with an aqueous solution of soda (NaOH) and of potassium metavanadate (KVO.sub.3) and/or ammonium metavanadate (NH.sub.4 VO.sub.3). Whereafter the earth is thickened, for example with carboxymethyl cellulose (CMC) or with a polyacrylamide. The paste is then extruded, thereby obtaining particles of different shape (hollow or solid cylinders, polylobed cylinders, optionally having helical grooves and so on). Prior to use, the catalyst is activated by a SO.sub.2 stream and a final activation is carried out by the same process fluid, containing SO.sub.2, SO.sub.3, O.sub.2 and N.sub.2, at the reaction temperature (350.degree.-650.degree. C.). During the oxidation of SO.sub.2 to SO.sub.3 the activated material is in the form of a liquid film on the surface of the carrier's pores. The average life of these catalysts is between 5 and 10 years with a gradual decrease to the vanadium content (for instance from about 7% to about 5% by weight). The possibility of regenerating and reusing the exhausted catalyst would be, therefore, an advantage of great importance for the industry. So far, however, it was not possible to carry into effect any of the alternatives.
According to a first alternative [see Journal of Catalysis 43, 243-251 (1976)], the exhausted catalyst should be attacked with a strong acid (HCl), capable of solubilizing the vanadium. The same vanadium should be recovered by extraction. This plan was not carried into effect, because the insoluble siliceous gangue did tenaciously retain a non-negligible vanadium content. The difficulties connected with a separation from the gangue have proved so far insurmountable, at least from an industrial viewpoint.
A second alternative (see again the above-cited article in the Journal of Catalysis) comprises a strong heating of the exhausted catalyst (at 500.degree. C.), to remove arsenic and other poisons, followed by a grinding of the calcination residue. It is then necessary to knead the ground catalyst with sulphur and with a solution of ammonium sulphate, whereafter an activation with air at 750.degree.-800.degree. C. is carried. However, this method too is not free from drawbacks. The mechanical resistance (to compression), the resistance to ageing and the abrasion resistance are not fully satisfactory. Furthermore, the SO.sub.2 conversion yields are often unacceptably decreasing.
A last alternative is to re-use an exhausted catalyst having a very high silica content (higher than 80% by weight). In such a case, however, as a consequence of a too prolonged exploitation of the catalyst, a consirable amount of the catalytic activity is irremediably lost.
The Applicant has now perfected an excellent catalyst, which does not exhibit the abovesaid shortcomings and can be prepared in an utmost simple and rapid way, starting from a previously exploited and already exhausted catalyst.